This invention relates to the casting of steel strip.
It is known to cast metal strip by continuous casting in a twin roll caster. In this technique molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the achievement of sufficiently rapid and even cooling of metal over the casting surfaces of the rolls. In particular it has proved difficult to obtain sufficiently high cooling rates for solidification onto casting rolls with smooth casting surfaces and it has therefore been proposed to use rolls having casting surfaces which are deliberately textured by a regular pattern of projections and depressions to enhance heat transfer and so increase the heat flux achieved at the casting surfaces during solidification.
Our U.S. Pat. No. 5,701,948 discloses a casting roll texture formed by a series of parallel groove and ridge formations. More specifically, in a twin roll caster the casting surfaces of the casting rolls may be textured by the provision of circumferentially extending groove and ridge formations of essentially constant depth and pitch. This texture produces enhanced heat flux during metal solidification and can be optimized for casting of steel in order to achieve both high heat flux values and a fine microstructure in the as-cast steel strip. Essentially when casting steel strip, the depth of the texture from ridge peak to groove root should be in the range 5 microns to 50 microns and the pitch of the texture should be in the range 100 to 250 microns for best results. For optimum results it is preferred that the depth of the texture be in the range 15 to 25 microns and that the pitch be between 150 and 200 microns.
Although rolls with the texture disclosed in U.S. Pat. No. 5,701,948 have enabled achievement of high solidification rates in the casting of ferrous metal strip it has been found that they exhibit a marked sensitivity to the casting conditions which must be closely controlled to avoid two general kinds of strip defects known as “crocodile-skin” and “chatter” defects. More specifically it has been necessary to control crocodile-skin defects by the controlled addition of sulphur to the melt and to avoid chatter defects by operating the caster within a narrow range of casting speeds.
The crocodile-skin defect occurs when δ and γ iron phases solidify simultaneously in shells on the casting surfaces of the rolls in a twin roll caster under circumstances in which there are variations in heat flux through the solidifying shells. The δ and γ iron phases have differing hot strength characteristics and the heat flux variations then produce localized distortions in the solidifying shells which come together at the nip between the casting rolls and result in the crocodile-skin defects in the surfaces of the resulting strip.
A light oxide deposit on the rolls having a melting temperature below that of the metal being cast can be beneficial in ensuring a controlled even heat flux during metal solidification on to the casting roll surfaces. The oxide deposit melts as the roll surfaces enter the molten metal casting pool and assists in establishing a thin liquid interface layer between the casting surface and the molten metal of the casting pool to promote good heat flux. However, if there is too much oxide build up the melting of the oxides produces a very high initial heat flux but the oxides then resolidify with the result that the heat flux decreases rapidly. This problem has been addressed by endeavoring to keep the build up of oxides on the casting rolls within strict limits by complicated roll cleaning devices. However, where roll cleaning is non-uniform there are variations in the amount of oxide build up with the resulting heat flux variations in the solidifying shells producing localized distortions leading to crocodile-skin surface defects.
Chatter defects are initiated at the meniscus level of the casting pool where initial metal solidification occurs. One form of chatter defect, called “low speed chatter”, is produced at low casting speeds due to premature freezing of the metal high up on the casting rolls so as to produce a weak shell which subsequently deforms as it is drawn further into the casting pool. The other form of chatter defect, called “high speed chatter”, occurs at higher casting speeds when the shell starts forming further down the casting roll so that there is liquid above the forming shell. This liquid, which feeds the meniscus region, cannot keep up with the moving roll surface, resulting in slippage between the liquid and the roll in the upper part of the casting pool, thus giving rise to high speed chatter defects appearing as transverse deformation bands across the strip.
Moreover, to avoid low speed chatter on the one hand and high speed chatter on the other, it has been necessary to operate within a very narrow window of casting speeds. Typically it has been necessary to operate at a casting speed within a narrow range of 30 to 32 meters per minute. The specific speed range can vary from roll to roll, but in general the casting speed must be well below 40 meters per minute to avoid high speed chatter.
We have now determined that it is possible to produce a roll casting surface which is much less prone to generation of chatter defects and which enables the casting of steel strip at casting speeds well in excess of what has hitherto been possible without producing strip defects. Moreover, the casting surface provided in accordance with the invention is also relatively insensitive to conditions causing crocodile-skin defects and it is possible to cast steel strip without crocodile-skin defects.
According to the invention there is provided a method of continuously casting steel strip comprising the steps of
supporting a casting pool of molten steel on one or more chilled casting surfaces textured by a random pattern of discrete projections wherein at least some of the projections include peaks having an average surface distribution of between 5 and 200 projections per mm2; and
moving the chilled casting surface or surfaces to produce a solidified strip moving away from the casting pool.
The random pattern of discrete projections is such as are produced by grit blasting the casting surface as hereinafter described. As noted, the discrete projections may have peaks. These peaks may be pointed peaks, but generally because of the nature of their formation, such discrete projections do not have such pointed peaks. It has been found that the peaks of the discrete projections have flat areas of typically 100 to 400 square microns due to the nature of formation, e.g., grit blasting. The discrete projections may have peaks that have an average distribution of between 5 and 200 peaks per mm2, with average peak distributions above 100 peaks per mm2 used with higher casting speeds. The average height of the discrete projections may be at least 10 microns and may also be at least 20 microns.
Therefore, in another illustrative embodiment, the average height of the discrete projections is at least 10 microns.
In yet another illustrative embodiment, the average height of the discrete projections is at least 20 microns.
Illustratively, the strip is moved away from the casting pool at a speed of more than 40 meters per minute. For example, the method permits the strip to be moved away at a speed of between 50 and 65 meters per minute.
The molten steel may be a low residual steel having a sulphur content of not more than 0.025%.
In another illustrative embodiment, at least some of the projections include peaks having an average surface distribution of between 10 and 100 peaks per mm2 and an average height of at least 10 microns. It will be appreciated that the average height of the discrete projections may be at least 20 microns in an alternative embodiment. Furthermore, the strip may be moved away from the casting pool at a speed of more than 40 meters per minute. For example, this illustrative method permits the strip to be moved away at a speed of between 50 and 65 meters per minute. Also in this illustrative embodiment, the molten steel may be a low residual steel having a sulphur content of not more than 0.025%.
The method of the present invention may be carried out in a twin roll caster.
Accordingly the invention further provides a method of continuously casting steel strip of the kind in which molten metal is introduced into the nip between a pair of parallel casting rolls via a metal delivery nozzle disposed above the nip to create a casting pool of molten steel supported on casting surfaces of the rolls immediately above the nip and the casting rolls are rotated to deliver a solidified steel strip downwardly from the nip, wherein the casting surfaces of the rolls are each textured by a random pattern of discrete projections, at least some of which include peaks having an average surface distribution of between 5 and 200 peaks per mm2 and an average height of at least 10 microns. In an alternative embodiment, at least some of the projections may include peaks having an average surface distribution of between 10 and 100 peaks per mm2. In an alternative embodiment the discrete projections may have an average height of at least 20 microns.
The invention further extends to apparatus for continuously casting steel strip comprising a pair of casting rolls forming a nip between them, a molten steel delivery nozzle for delivery of molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on casting roll surfaces immediately above the nip, and a roll drive that moves the casting rolls in counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip, wherein the casting surfaces of the rolls are each textured by a random pattern of discrete projections, at least some of which include peaks having an average surface distribution of between 5 and 200 peaks per mm2. In another illustrative embodiment, at least some of the projections may include peaks having an average surface distribution of between 10 and 100 peaks per mm2. Illustratively, the discrete projections may have an average height of at least 10 microns. In another illustrative embodiment, the discrete projections may have an average height of at least 20 microns.
A textured casting surface in accordance with the invention can be achieved by grit blasting the casting surface or a metal substrate which is protected by a surface coating to produce the casting surface. For example each casting surface may be produced by grit blasting a copper substrate which is subsequently plated with a thin protective layer of chrome. Alternatively, the casting surface may be formed of nickel in which case the nickel surface may be grit blasted and no protective coating applied.
The required texture of the or each casting 5 surface may alternatively be obtained by deposition of a coating onto a substrate. In this case the material of the coating may be chosen to promote high heat flux during metal solidification. Said material may be a material which has a low affinity for the steel oxidation products so that wetting of the casting surfaces by those deposits is poor. More particularly the casting surface may be formed of an alloy of nickel chromium and molybdenum or alternatively an alloy of nickel molybdenum and cobalt, the alloy being deposited so as to produce the required texture.